Apparatus for melting metal and method for manufacturing metal

ABSTRACT

An apparatus for melting metals comprises a hearth for melting a metal raw material, a mold for forming an ingot by cooling the molten metal therein, the mold having a bottom, and a base unit for pulling down the ingot and provided at the bottom of the mold. The base unit has a surface provided with a concave portion at an optional location, and the surface of the base unit surrounding the concave portion is inclined toward the concave portion.

TECHNICAL FIELD

The present invention relates to an apparatus for melting metalmaterials and relates to a method for producing metals using theapparatus. Specifically, the present invention relates to a techniquefor preventing melting failure and surface defects, which may form on aningot portion at the initial melting stage for the production of aningot of the metal material in an electron-beam melting furnace or aplasma-arc melting furnace.

BACKGROUND ART

It is known, as a technique for producing an ingot, that a meltingfurnace provided with a water-cooled copper crucible may be used, andthat a perpendicularly movable water-cooled base unit may be disposed atthe bottom of the crucible. While an electron beam is emitted on a metalmaterial in a vacuum atmosphere, and a molten metal material is pouredor is dropped into the water-cooled base unit, the water-cooled baseunit is continuously or intermittently pulled down so as to produce aningot of the metal material.

The molten metal contacts the water-cooled base unit and is solidifiedfrom the portion at the start of melting of the metal material. Themolten metal is piled up and is solidified on the approximately entiresurface of the water-cooled base unit after the molten metal is fed fora certain time. Then, an electron beam is emitted on the metal materialpiled up on the water-cooled base unit so as to melt the entire surface,and the water-cooled base unit is pulled down. Since the level of themolten metal is lowered according to the pulling down of thewater-cooled base unit, a molten metal melted by an electron beam isfurther fed into the water-cooled cooper crucible. Thus, by pulling downthe water-cooled base unit and continuously feeding a molten metal,ingots may be successively produced.

The water-cooled base unit has a flat surface, and the molten metal thatis poured thereinto is solidified in a short time at the location atwhich the molten metal falls. When more molten metal subsequently fallson the solidified metal, the molten metal flows in any direction and issolidified in a short time. In the initial stage of the melting, thefalling of the molten metal into the water-cooled base unit andsolidification of the molten metal as described above occur repeatedly.Therefore, a bottom portion of the ingot produced in the initial stageof the melting (hereinafter called an “initial molten metal portion”),specifically, an ingot portion contacting the water-cooled base unit, isformed with portions that are insufficiently melted and have surfacedefects. Since there may be a case in which the portions that areinsufficiently melted and have surface defects will be an obstacle inthe subsequent working process, the portions that are insufficientlymelted and that have surface defects are removed in advance by machiningor cutting. This process reduces the process yield of the ingot andthereby requires improvement.

A technique for preventing the melting failure and the surface defectsis known. In this technique, an electron beam is emitted on a moltenmetal immediately after the molten metal falls on the water-cooled baseunit so as to maintain the initial molten metal portion in a meltedstate. In this case, the electron beam is emitted on a location in whichthe molten metal fell on the water-cooled base unit so as to form amolten metal and to maintain the melted state. Therefore, the fallenmolten metal solidifies while an irradiation location of the electronbeam thereto is adjusted.

Regarding this problem, a technique for preventing the solidification ofthe initial molten metal portion has been proposed. In this technique,an electron beam is emitted on a wide surface including the fallenmolten metal on the water-cooled base unit, thereby temporarilypreventing a part of the initial molten metal from solidifying. In thiscase, the electron beam is emitted on the surface of the water-cooledbase unit which is not covered with the molten metal, and there may be acase in which the surface of the water-cooled base unit is damaged bymelting. Therefore, this technique requires improvement.

As a method for solving the above problems, a technique is disclosed inJapanese Unexamined Patent Application Publication No. 2000-274957, forexample. In this method, a metal block material having the same grade asthat of a metal to be melted is disposed on the water-cooled base unit,and melting is started after the metal block material is irradiated withan electron beam so as to form sufficient molten metal surface.According to this method, the charged molten metal does not solidifybefore an electron beam is emitted thereon, and the water-cooled baseunit is not damaged even when the electron beam is emitted on an areaincluding the fallen molten metal. The patent document discloses only amethod in which feeding of a molten metal into a mold is started after amolten metal surface is formed on the top of the metal block that ispreviously disposed on the water-cooled base unit. On the other hand,the patent document does not disclose a method for forming an initialmolten metal portion. Accordingly, the portion corresponding to themetal block disposed on the water-cooled base unit should be removed,whereby the resultant yield is decreased, and the additional costs areincurred for the removal.

Moreover, a technique for continuously casting an ingot is disclosed inJapanese Unexamined Patent Application Publication No. 2000-153345, forexample. In this technique, a water-cooled base unit having awedge-shaped portion that is engageable with an ingot is provided on thebottom of a mold. The wedge-shaped portion and an initial molten metalportion of an ingot are engaged when the molten metal falls on thewater-cooled base unit, and the water-cooled base unit is pulled downafter the ingot is solidified.

Japanese Unexamined Patent Application Publication No. 2000-153345 doesnot disclose a method for forming an initial molten metal portion and amethod for preventing portions that are insufficiently melted and havesurface defects occurring on an initial molten metal portion. Asdescribed above, there have been no effective methods for solving theproblem relating to the formation of an initial molten metal portion inthe production of ingots.

DISCLOSURE OF THE INVENTION

The present invention has been completed in view of the abovecircumstances. An object of the present invention is to provide a methodfor solving the above problem remaining in the conventional techniquesand to produce ingots on the more superior conditions of the processyields. That is, the present invention provides a method for melting ametal material while preventing melting failure and surface defects,which will form on an initial molten metal portion at the start ofmelting.

The inventors performed intensive research so as to solve the problemremaining in the above conventional techniques. As a result, theinventors found the following, and the present invention has therebybeen completed. That is, a melting furnace comprising a water-cooledcopper mold is used, and the bottom of the water-cooled copper mold isdisposed with a water-cooled base unit for pulling down an ingot that isproduced. The water-cooled base unit is provided with a concave portionon the surface thereof and is provided with an incline on the surfacesurrounding the concave portion of the base unit. Therefore, moltenmetal that falls or drops into the mold can be collected at the concaveportion provided at the water-cooled base unit.

That is, the present invention provides an apparatus for melting metalscomprising a hearth for melting a metal raw material and a mold intowhich a molten metal is poured so as to form an ingot. The mold has abottom and is provided with a base unit for pulling down the ingot atthe bottom. The base unit has a surface and is provided with a concaveportion at an optional location of the surface, and the surfacesurrounding the concave portion of the base unit is inclined toward theconcave portion.

Moreover, in the present invention, another concave portion is providedto the concave portion at the bottom on the surface of the base unit.The base unit to be pulled down the ingot has a separable structure sothat the ingot formed thereon can be pulled out.

According to the apparatus for melting metals of the present inventionhaving the above structure, the surface of the base unit arranged at thebottom of the mold is provided with a concave portion, and the surfaceof the base unit other than the concave portion is inclined toward theconcave portion. Therefore, a molten metal, which is poured from thehearth and reaches the base unit at the beginning, flows into theconcave portion first according to the incline. Then, the followingmolten metals fed are filled in the mold in order from the concaveportion and are solidified by cooling. As a result, surface defects andthe portions caused by the insufficient melting conditions areeffectively reduced and may not occur on the initial molten metalportion of an ingot that is produced.

The molten metal can be collected at the bottom of the concave portionon the base unit surface, thereby further which reduces the surfacedefects and the portions caused by the insufficient melting conditions.

Moreover, since the base unit for pulling down the ingot has a separablestructure, the ingot produced on the base unit can be easily pulled outafter melting of the metal raw material is completed.

In the conventional methods, the melting failure and the surface defectsof the initial molten metal portion are removed by cutting. In contrast,in the present invention, the surface defects and the portions that areinsufficiently melted hardly occur on the initial molten metal portionas described above. Therefore, the process yield of the ingot can beimproved compared to the conventional methods.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic sectional view of an electron-beam meltingapparatus of the present invention.

FIG. 2 is a schematic sectional view showing a modification example of awater-cooled base unit of the present invention.

FIG. 3 is a schematic sectional view showing another modificationexample of a water-cooled base unit of the present invention.

FIG. 4 is a schematic sectional view showing another modificationexample of a water-cooled base unit of the present invention.

FIG. 5 is a schematic sectional view showing another modificationexample of a water-cooled base unit of the present invention.

FIG. 6 is a schematic sectional view showing a conventional water-cooledbase unit.

EXPLANATION OF REFERENCE NUMERALS

1 denotes a device for feeding raw materials, 2 denotes a titaniumsponge, 3 denotes a cold hearth, 4 denotes a molten metal, 5 denotes awater-cooled copper mold, 6 denotes an electron beam gun, 7 a to 7 edenote water-cooled base units (examples of the present invention), 7 fdenotes a water-cooled base unit (example used in conventionaltechniques), 71 denotes an inclined portion of a base unit, 72 denotes aconcave portion of a base unit, 73 denotes an inverse tapered portion,74 denotes a bottom of a base unit, 75 denotes an inclined bottomportion, 76 denotes a horizontal bottom portion, and 77 denotes aninclined bottom portion.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be described indetail with reference to the figures.

FIG. 1 shows a preferred embodiment for producing a titanium ingot froma titanium sponge, which is a raw material to be melted, by using anelectron-beam cold hearth melting furnace. The reference numeral 1indicates a device for feeding raw materials, by which a titanium sponge2 that is a raw material is fed. In downstream of the device 1 forfeeding raw materials, there is provided a cold hearth 3 which is madeof a water-cooled copper and contains a molten metal 4 including moltentitanium. A water-cooled copper mold 5 is provided downstream of thecold hearth 3, and the whole of the cold hearth 3 is obliquely arranged,whereby the molten metal 4 can be poured from the cold hearth 3 into thewater-cooled copper mold 5. An electron beam gun 6 is provided over thecold hearth 3 and the water-cooled copper mold 5, and an electron beamis emitted therefrom so as to melt the titanium sponge 2.

The water-cooled copper mold 5 is provided with a water-cooled base unit7 a at the bottom. As shown in FIG. 1, a concave portion 72 of the baseunit is formed at the center of the water cooled base unit 7 a, and theperiphery thereof is formed with an inclined portion 71 of the base unitthat is inclined toward the concave portion 72 of the base unit. Theconcave portion 72 of the base unit comprises an inverse tapered portion73, which forms the side wall of the concave portion 72 of the baseunit, and a bottom 74 of the base unit. As shown in FIG. 1, the inversetapered portion 73 is inclined from the perpendicular direction so thatthe inverse tapered portion 73 and the bottom 74 of the base unit forman acute angle.

Then, operation methods of the hearth electron-beam melting furnace willbe described as follows. The bottom of the cold hearth 3 is disposedwith a solid layer of a titanium called a “skull” (not shown in thefigure) before the melting of the titanium is started, and an electronbeam is emitted on the skull so as to form a molten metal 4. Then, atitanium sponge 2 is fed into the cold hearth 3 by the device 1 forfeeding raw materials, and the electron beam is emitted on the titaniumsponge 2 so that the titanium sponge 2 is melted and is mixed with themolten metal 4. After the molten metal 4 is flown and is refined in thecold hearth 3, the molten metal 4 is poured into the water-cooled coppermold 5.

The molten metal 4 poured into the water-cooled copper mold 5 reachesthe water-cooled base unit 7 a that is disposed at the bottom of thewater-cooled copper mold 5. A portion of the molten metal 4, whichreached the water-cooled base unit 7 a, reaches the concave portion 72of the base unit and is solidified after the molten metal 4 is cooled bythe bottom portion 74 of the base unit for a while. The other portion ofthe molten metal 4, which reached the water-cooled base unit 7 a,reaches the inclined portion 71 of the base unit and rapidly flows intothe concave portion 72 of the base unit according to the incline of theinclined portion 71 of the base unit. Then, the molten metal 4 issolidified in the same way as that of the above molten metal 4. Thus,the molten metal that reaches any portion of the base unit flows intothe concave portion 72 of the base unit, thereby being solidified. Themolten metal 4 is further fed into the water-cooled copper mold 5 untilthe molten metal 4 approximately covers the inclined portion 71 of thebase unit, and the molten metal 4 is solidified so as to form an initialmolten metal portion of titanium ingot. In this case, the molten metalmay be solidified in a short time immediately after the molten metal isfed into the concave portion 72 of the base unit at the beginning,because the water-cooled base unit is not sufficiently heated by theheat of the molten metal itself and is in a low-temperature condition.Therefore, the output level of the electron beam for irradiation ispreferably increased.

After an initial molten metal portion is formed so as to cover theconcave portion 72 of the base unit and the inclined portion 71 of thebase unit, the water-cooled base unit 7 a is pulled down so as to expandthe space over the water-cooled copper mold 5, and the molten metal 4 isfurther fed to the space. Thus, the water-cooled base unit 7 a is pulleddown while the molten metal 4 is fed into the water-cooled copper mold5, whereby the molten metal is cooled and is solidified in order fromthe lower portion to the upper portion of the mold. As a result,titanium ingots can be successively produced. When the water-cooled baseunit 7 a is pulled down, since the inverse tapered portion 73 in theconcave portion 72 of the base unit engages with the initial moltenmetal portion of titanium ingot, the initial molten metal portion andthe water-cooled base unit 72 will not be separated, and the titaniumingot can be pulled down.

Other preferred modification examples of the structural member of thepresent invention will be described.

The inclined portion of the base unit on the water-cooled base unit ofthe present invention is preferably inclined from the periphery towardthe center thereof.

The inclined portion of the base unit is preferably formed to have aninclination angle of 2 to 10° with respect to the horizontal surfacewhen a titanium is used as a molten metal. By forming the inclinedportion of the base unit to have an inclination angle of the aboverange, the molten metal which is fed can be uniformly poured into thecenter portion of the water-cooled base unit. When the inclination angleis less than 2°, it is difficult to rapidly pour the molten metal tooptional locations of the water-cooled base unit due to the viscosity ofthe molten titanium. When the inclination angle is more than 10°, theratio of the initial molten metal portion to the ingot produced isincreased, whereby the process yield may be decreased. It should benoted that the lower limit of 2° of the inclination angle range iseffective when titanium is to be ingoted, and the lower limit isselected according to the viscosity of a metal to be ingoted.

As another preferred embodiment of the present invention, the followingsteps may be used. Titanium sponge is previously disposed on thewater-cooled base unit, and an electron beam is emitted thereon so as tomelt the titanium sponge. Then, the molten titanium sponge is pouredinto the concave portion of the base unit so as to form an initialmolten metal portion. According to this embodiment, the titanium spongecovers the water-cooled base unit, whereby the water-cooled base unit isnot damaged by irradiation by the electron beam in forming an initialmolten metal portion.

FIG. 2 shows another preferred embodiment relating to the presentinvention. That is, FIG. 2 shows a modification example of thewater-cooled base unit 7 a of FIG. 1. In the water-cooled base unit 7 b,the concave portion 72 of the base unit is not provided with ahorizontal bottom, but is instead provided with an inclined portion 75that is inclined toward the center portion. According to thewater-cooled base unit 7 b having such a structure, the molten metalpoured thereinto first is led to the center of the concave portion ofthe base unit and can be solidified in sequence while preventing themolten metal from solidifying at random portions.

FIG. 3 shows another preferred embodiment of the water-cooled base unit7 a. The water-cooled base unit 7 c further comprises an inclinedportion 75 and a horizontal portion 76. By forming such a concaveportion, the bottom of the initial molten metal portion of an ingot thatis produced can be formed with a convex portion having a gentle curve.Therefore, the initial molten metal portion produced by using thewater-cooled base unit 7 c can be handled more easily than that producedby using the water-cooled base unit 7 b shown in FIG. 2.

FIG. 4 shows a further preferable embodiment of the water-cooled baseunit 7 a. The water-cooled base unit 7 d comprises a concave portion 72of the base unit having a bottom 77, and the bottom 77 is flat and isinclined toward the right of the paper surface. According to thewater-cooled base unit 7 d having such a structure, the molten metalthat is poured from the hearth into the base unit can be led along theinclination direction of the bottom of the concave portion 72 of thebase unit. As a result, the molten metal is solidified in order from thelowest end portion of the bottom 77, whereby macroscopic defects do notform on the solidified portion of the molten metal, and an ingot with agood quality can be produced.

FIG. 5 is a plan view showing a water-cooled base unit that relates tothe present invention and is observed from the top, and the water-cooledbase unit has a rectangular shape. That is, an ingot that is produced inthis embodiment has a rectangular shape in cross section. In thisembodiment, the concave portion 72 of the base unit is formed into atrapezoidal shape. The line L-L′ indicates a separation line of themold. The water-cooled base unit 7 e has a structure that can beseparated into two portions (7 e-a and 7 e-b) at the separation line.Specifically, the lower base unit of the trapezoidal shape portion ispreferably arranged at the side of the separation line. In this case,the lower base unit is longer than the upper base unit, and each sideconnecting the upper base unit and the lower base unit preferably has anangle in a range of 30 to 60° with respect to the horizontal line from apractical point of view.

According to such an arrangement, after a predetermined amount of aningot is produced, the water-cooled base unit 7 e-a is separated fromthe water-cooled base unit 7 e-b, and the ingot piled up on the uppersurface of the water-cooled base unit 7 e is slid in a directionperpendicular to the line L-L′. As a result, the ingot formed with afitting portion that corresponds to the concave portion can be separatedfrom the water-cooled base unit 7 e.

The separable structure of the water-cooled base unit as described aboveis preferably applied to the above water-cooled base units 7 a to 7 d.Such a separable structure facilitates pulling out of the ingot, whichis produced, from the water-cooled base unit.

The horizontal cross section of the water-cooled base units 7 a to 7 emay be formed into a circular shape as well as the above rectangularshape so as to form an ingot having a circular shape.

One of the water-cooled base units 7 a to 7 e as described above isdisposed in the water-cooled copper mold 5 before melting of the metalraw material. Therefore, a molten metal poured from the hearth isappropriately led to the concave portion, whereby the initial moltenmetal portion and the water-cooled base unit can be strongly engaged.Moreover, the formed ingot can be reliably pulled out from thewater-cooled base unit after the melting is completed.

EMBODIMENTS

Hereinafter, the present invention will be specifically described withreference to embodiments. The embodiments are example of the preferredembodiment of the present invention, and the present invention is notlimited thereto.

First Embodiment

The water-cooled base unit 7 c shown in FIG. 3 was mounted to thewater-cooled copper mold 5 of the electron-beam melting apparatus inFIG. 1. Then, the cold hearth 3 was irradiated with an electron beam andwas fed with a titanium sponge 2 so as to form a molten metal 4. Themolten metal 4 was fed into the water-cooled copper mold 5, and atitanium ingot was produced. The ingot produced was cooled and wasseparated from the water-cooled base unit 7 c by cutting. The structureat the bottom of the ingot that was cut had a good quality and had thesame quality as that of a material that can be used for a hot forging.The process yield of the collected titanium ingot was 98% with respectto the theoretical yield that was calculated from the input amount ofthe raw material.

Second Embodiment

An ingot was produced under the same conditions as those of the FirstEmbodiment, except that the water-cooled base unit 7 d was used insteadof the water-cooled base unit 7 c. As a result, the process yield of thecollected titanium ingot was 98% with respect to the theoretical yieldthat was calculated from the input amount of the raw material.

Comparative Embodiment

An ingot was produced under the same conditions as those of the FirstEmbodiment, except that the conventional water-cooled base unit 7 fshown in FIG. 6 was used. The titanium ingot that was produced wasseparated from the water-cooled base unit by cutting. The cut surface ofthe separated titanium ingot was examined, and there were portions whichwere not sufficiently melted, and the portions were thereby cut off. Asa result, the net process yield of the titanium ingot was only 95%.

INDUSTRIAL APPLICABILITY

According to the method for melting a metal material by using anelectron beam of the present invention, melting failure and surfacedefects of the initial molten metal portion of an ingot can be reduced.Therefore, the process yield in producing an ingot can be furtherimproved.

1. An apparatus for melting metals comprising: a hearth for melting ametal raw material; a mold for forming an ingot by cooling the moltenmetal therein, the mold having a bottom; and a base unit for pullingdown the ingot and provided at the bottom of the mold, wherein the baseunit has a surface provided with a concave portion at an optionallocation, the concave portion comprises a bottom and a side wall portionthat is inclined from the perpendicular direction so as to form an acuteangle with the bottom, and the surface of the base unit surrounding theconcave portion is inclined toward the concave portion.
 2. The apparatusfor melting metals according to claim 1, wherein the surface of the baseunit inclined toward to the concave portion has an inclination angle of2 to 10°.
 3. (canceled)
 4. The apparatus for melting metals according toclaim 1, wherein another concave portion is provided further to theconcave portion at the bottom.
 5. The apparatus for melting metalsaccording to claim 4, wherein the concave portion provided at the bottomof the concave portion has a bottom surface, and the bottom surface isinclined in any direction with respect to a horizontal surface.
 6. Theapparatus for melting metals according to claim 1, wherein the base unitfor pulling down the ingot has a separable structure so as to allow theingot formed thereon to be pulled out.
 7. The apparatus for meltingmetals according to claim 1, wherein the base unit comprises awater-cooled copper.
 8. The apparatus for melting metals according toclaim 1, wherein the apparatus for melting metals comprises one of anelectron-beam melting furnace and a plasma-arc melting furnace.
 9. Theapparatus for melting metals according to claim 1, wherein the metal isselected from the group consisting of titanium, zirconium, and tantalum.10. A method for producing metals by using the apparatus for meltingmetals according to claim
 1. 11. The method for producing metalsaccording to claim 10, wherein the metal is selected from the groupconsisting of titanium, zirconium, and tantalum.